The lower level smoke control plenum in a new academic building at a major university needed protection to prevent premature deterioration from any condensation on the plenum walls in addition to meeting Class A flame and smoke requirements. An initial solution called for lining the interior side of the plenum with sheet steel. With the presence of service lines blocking access, the application of sheet steel would have added extreme difficulty to the application, almost ensuring trouble achieving successful implementation.
Interior gypsum wallboard was used to construct the plenum. The building and mechanical system design conditions called for the gypsum wallboard surface temperature to be maintained above the airstream dew-point temperature with an air temperature range of 50°F (10°) to 125°F (52°C). While the airstream and surface temperature restrictions for the use of gypsum wallboard in plenums are intended to prevent premature deterioration of the gypsum wallboard, plenums connected to the exterior enclosure can move moist air through these assemblies. Negative pressure in the plenum can suck saturated air from any building envelope leaks out of the wall cavity into the plenum where it can condense on piping, ductwork and anything else it contacts that has a surface temperature below its dew-point.
To provide the protection required, Tremco’s ExoAir® 230 Fluid-Applied Vapor-Permeable Air Barrier was proposed to seal the interior walls of the plenum from air infiltration and exfiltration, while allowing vapor molecules to pass through so they do not get trapped within the wall. Trapped moisture vapor could cause condensation to occur within the wall cavity which can lead to structural deterioration and shortened structure life. With exceptional movement capability (+100/-50) and documented compatibility with Tremco air barriers, Tremco's Dymonic® 100 Polyurethane Sealant was proposed to treat perimeter joints and ensure a smooth transition from one plane to another and at penetrations of the interior grade gypsum wallboard.
Tremco Testing Validates Design
A series of laboratory tests were performed at the Tremco Sustainable Building Solutions Test Facility to determine air leakage resistance, water leakage resistance and durability of the air barrier coating. Architectural Testing, Inc., a subsidiary of Intertek, witnessed the testing and validated the results. Since workmanship and installation details are so critical to determining performance, field testing of the installation was also required.
ExoAir® 230 had already been tested in assemblies according to NFPA 285 Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components and had passed. It also had GREENGUARD Children & Schools certification.
The 8’ x 8’ test assembly was constructed using a wood buck with ½” sheet rock attached to the exterior with self-tapping tech screws spaced evenly every 8” around the perimeter and 12” up the studs. The sheeting installation allowed for an 8’ x 1/8” horizontal board joint at the center, which was taped and then treated with joint compound over the top. Dymonic 100 Polyurethane Sealant was used to treat the perimeter joints to the wood buck. All fasteners were detailed with joint compound. After the joint compound was fully cured, 75 wet mils of ExoAir 230 was applied to the assembly.
Based on having passed all ASTM E283, ASTM E331 and ASTM E2357 test requirements, this approach to plenum protection received pre-approval for use in the installation. Upon completion of the installation, on-site testing was required to verify successful implementation and performance in the field.
ATI Report E331
ATI Report E2357
Conclusion & ResultsASTM E2357 is intended to simulate the performance of various air barrier materials and accessories when combined into an assembly. Special equipment at the Tremco test facility creates effects of various levels of sustained wind loads, cyclic wind loads and gust wind loads at both positive and negative sides of the wall, simulating real-life conditions. The amount of air flow that penetrates through the assembly at different levels of pressure is the basis for its air leakage ratings. Maintaining airtight requirements on the interior of the plenum minimizes the potential for moisture-laden air to condense on the surface and lead to problems down the line. Performance requirements in the specifications for the plenum application called for air barrier assembly air leakage of a maximum 0.04 cfm/sq. ft. of surface area at 1.57 lbf/sq. ft. (0.2 L/s x sq. m of surface area at 75 Pa). The test assembly performed well below the acceptable standard for air leakage specified.
The City of Los Angeles Building & Safety Department required that the plenum used for smoke control be protected from the outside elements. ASTM E331 testing provided assurance that the assembly would serve as an effective drainage plane to discharge any incidental condensation or moisture in the event of water penetration. A 15-minute Water Exposure Test in accordance with the test method, with a pressure differential of 137 Pa (2.86 psf), met the performance requirements as no water penetration was detected.
The proficient installation of the air barrier assembly is critical in determining the air leakage of the system. In addition to impacting the life cycle of the duct system, heating and cooling loads as well as occupant comfort and other expected benefits of the air barrier system are negatively affected when materials are not applied with the attention and skill required. One of the largest fire protection and life safety engineering and consulting firms with offices worldwide witnessed door fan testing of the modified lower level smoke control make-up air plenum serving the atrium smoke control system of the four-story building plus basement and verified that allowable leakage factors were met. Prior to coating, the duct could not be pressurized to required pressure levels for testing due to air leakage. After the ExoAir 230 coating was applied, the air leakage was reduced sufficiently to not only allow sufficient pressurization to occur, but to perform within the limits of Section 909 of the 2011 LABC, City of Los Angeles requirements.